1. Field of the Invention
The present invention relates to light diffusion sheets which have directional light diffusion function to allow transmitted rays of light to diffuse while collecting to a side in a normal direction and which are particularly suitable for a backlight of a liquid crystal display device, and backlight units in which this sheet is used.
2. Description of the Related Art
Liquid crystal display devices in widespread use have included a backlight system where light emission is executed by irradiating onto a liquid crystal layer from the back face. In such a type of display devices, a backlight unit which is an edge light type, an immediate beneath type or the like is provided to an under face side of the liquid crystal layer. Such a backlight unit 50 of an edge light type is equipped with a rod-shaped lamp 51 for use as a light source, an optical waveguide plate 52 having a square plate shape disposed so that the edge thereof abuts along the lamp 51, a light diffusion sheet 53 disposed to the front face side of the optical waveguide plate 52, and a prism sheet 54 disposed to the front face side of the light diffusion sheet 53, as fundamentally shown in FIG. 3 (a).
Referring to functions of this backlight unit 50, rays of incident light from the lamp 51 to the optical waveguide plate 52 are first reflected on reflection dots or a reflection sheet (not shown in the Figure) of the back face of the waveguide plate 52, and exit from the front face of the waveguide plate 52. The rays of light exited from the waveguide plate 52 enter into the light diffusion sheet 53, then are diffused by the light diffusion sheet 53 and exit from the front face of the light diffusion sheet 53. Thereafter, the rays of light exited from the light diffusion sheet 53 enter into the prism sheet 54, and exit as rays of light having a distribution representing a peak in a direction along a substantially normal line via a prism part 54a formed on the front face of the prism sheet 54.
Accordingly, the rays of light exited from the lamp 51 are diffused by the light diffusion sheet 53, and refracted by the prism sheet 54 so that they represent a peak in a direction along the substantially normal line, and illuminate the entire face of the liquid crystal layer on the front face side (not shown in the Figure). Meanwhile, although not shown in the Figure, a light diffusion sheet may be further disposed to the front face side of the prism sheet 54 for the purpose of: relaxation of light condensing properties of the prism sheet 54 as described above; protection of the prism part 54a; or prevention of the sticking between the prism sheet 54 and the liquid crystal panel such as a polarization plate.
The light diffusion sheet 53 to be disposed to the backlight unit 50 generally has a transparent substrate layer 56 made of a synthetic resin, and a light diffusion layer 57 overlaid on the front face of the substrate layer 56 as shown in FIG. 3 (b) (for example, see, JP-A Nos. H07-5305 and 2000-89007). In general, this light diffusion layer 57 includes resin beads 59 in a transparent resin binder 58, and the beads 59 exert the light diffusion function.
According to the conventional light diffusion sheet 53 described above, increase in the amount of incorporating the beads 59 and amount of the overlaid light diffusion layer 57 are generally suggested as means for improving light diffusibility, however, increase in amount of incorporating the beads 59 and amount of the overlaid light diffusion layer 57 may lead to difficulties in coating and decrease in transmittance of rays of light. In other words, according to belief of technical experts in connection with light diffusion sheet 53, light diffusibility, and coating facility and light transmissivity have been recognized as contradictory properties.
Taking into account of balance among light diffusibility, light transmissivity and coating facility, conventional general light diffusion sheets 53 based on the aforementioned belief have, polydisperse beads that have comparatively great mean particle size of approximately 20 μm, and have comparatively broad distribution of the particle size are used as the beads 59, while having the weight ratio of the beads 59 to the binder of approximately 1 and the amount of the overlaid light diffusion layer 57 of 15 to 20 g/m2.